BY JIM DEROGATIS POP MUSIC CRITIC

Midway through Sonic Youth’s third song at Metro on Saturday, just when you were thinking, “Gee, the addition of another guitarist/bassist (in the presence of former Chicagoan Jim O’Rourke) hasn’t really changed the band’s sound at all,” something clicked.

The long-running New York art-rockers’ sound suddenly opened up wide—like the moment in “The Wizard of Oz” when the film turns from black and white to Technicolor—as the beautiful, serpentine guitar lines of “The Empty Page” intertwined like copulating eels.

From that point on, while some of the old, feedback-drenched intensity was missing, it was a new and reinvigorated Sonic Youth that commanded the stage at the first of its two sold-out shows.

Like Metro itself (the club is celebrating its 20^th anniversary), Sonic Youth is an underground institution. While the band has never been as radical and innovative as its legions of fanzine boosters contend, for a time (from 1985’s “Bad Moon Rising” through 1992’s “Dirty”), it was one of the most gripping acts in rock, bringing bursts of sophisticated guitar terrorism into tuneful, hard-driving songs like “Death Valley ’69,” “Silver Rocket,” and “Dirty Boots.”

The quartet lost the plot circa 1995’s “Washing Machine” and its ill-fated headlining set on Lollapalooza. It stopped writing memorable songs and turned to vapid, meandering “experimentation,” emphasizing a snobbish avant-disregard for its audience that had always been there, but which now came to the fore.

The new album “Murray Street” is a return to form, and part of the credit is due to O’Rourke. In myriad projects of his own, the guitarist has been guilty of the same sort of artistic solipsism (another word would be “wankery”) that Sonic Youth fell prey to. But as a producer, he takes the opposite role, highlighting hooks that may have gotten lost in a band’s studio explorations. (He performed a similar service for Wilco on “Yankee Hotel Foxtrot,” which was a harsher, more disjointed effort before O’Rourke arrived as remixer.)

The rest of the credit belongs to the band itself, simply for returning to crafting actual tunes. Many of them are gorgeous and lulling new soundscapes in the tradition of “Pacific Coast Highway” from the heralded “Daydream Nation,” and this material sounded even better on stage than it does on the record.

Performing amid a simple but startlingly powerful light show and standing in front of a video backdrop of the crowd as seen from the stage (an effective idea I’d never seen before), the band highlighted new material like “Disconnection Notice,” “Rain On Tin,” and “Karen Revisited,” some of the highlights of a 12-song set (plus two well-deserved encores).

O’Rourke and Kim Gordon generally alternated on guitar and bass (though Gordon also took the opportunity to sing a few songs without playing anything), and on new material as well as old, they managed to find interesting spaces between Thurston Moore’s frenzied strumming (he still plays guitar with a drum stick, though he skipped the old screwdriver-in-the-neck trick) and Lee Ranaldo’s more structured lead lines.

Unfortunately, Steve Shelley remains the band’s weak point. He is a subtle and musical drummer, but what Sonic Youth has always needed is more powerful propulsion. (A less technical but more spirited player, Bob Bert remains the band’s definitive skinsman.)

With a throng of just-past-teenagers enthusiastically singing along to every word, you’d never have known that Sonic Youth, like Metro, has been around for 20 years. But in a set that celebrated a shining present while looking to a promising future, the group also acknowledged its past. Younger fans might not have recognized it, but the quintet ended its second encore with a triumphant version of “Making the Nature Scene,” a vintage oldie from its second album, 1983’s “Confusion Is Sex.”

Opening for Sonic Youth was the Ann Arbor trio Wolf Eyes, the sort of absurdly self-indulgent combo of would-be-Bohemian “artistes” that give underground music a bad name. For a group like this, insulting adjectives are taken as a compliment rather than a put-down, since their aim is to aggravate, so I refuse to indulge these smug jokers by troubling to say exactly how much their pointless noise sucked.